1. Field of the Invention
This invention relates to a speech processing system, and more particularly to a variable rate speech signal transmission method, by which the bandwidth of the speech signal is made variable, depending on the required transmission bit rate, and a system for realizing the method.
2. Description of the Related Art
In the case where speech signals are transmitted through a digital communication system, variable rate speech signal transmission techniques controlling the bandwidth of the signals, depending on the state of the transmission path, are desired.
Heretofore the variable rate coding of speech by the waveform coding method, by which the generation mechanism of speech is not taken into account, is discussed e.g. in the Bell System Technical Journal, Vol. 58, No. 3, March 1979, pp. 577-600. Further, the variable rate coding of speech by the source coding method, by which speed compression is effected by modeling the generation mechanism of the speech is described e.g. in Technical Research Report of the Institute of Electronics Communication Engineers of Japan, SP 86-48 (1986) pp.31-38.
However, by the former, the variable rate coding of speech by the waveform coding method, since the number of bits used for the quantization of each sample of the input waveform is changed, depending on the transmission rate, it is not possible to exclude the redundancy due to the speech generation mechanism, which is characteristic of the speech, and in a transmission system having a bit rate lower than 32k bits per second (bps) it is difficult to obtain practical compressed signals. On the other hand, by the latter, the variable rate coding of speech by the source coding method, although it is possible to obtain compressed speech signals bit for practical use for the bit rates lower than 32k bps, according to the coding method disclosed in the literature state above, e.g. for the bit rates higher than 8k bps the APC-MLQ (Adaptive Predictive Coding with Maximum Likelihood Quantization) is adopted and it is switched over for the bit rates lower than 7.2k bps to the hybrid coding combining the base band coding based on APC-MLQ algorithm and the high frequency regeneration method. According to this method, since the algorithm for the compressing processing is switched over depending on the bit rate, it has a problem that the construction of the coder and the decoder is too complicated.